Method, apparatus and system for high-speed transmission on fiber optic channel

ABSTRACT

Multi-carrier modulation fiber optic systems constructed using a series of electrical carriers, modulating the data on the electrical carriers and combining the carriers to form a wideband signal. The wideband signal can then be intensity modulated on a laser and coupled to a fiber optic channel. A receiver may then receive the laser signal from the fiber optic channel and convert it into an electrical signal. Multi-carrier modulation may be applied to existing fiber channels, which may be of lower quality. Existing fiber channels may have characteristics which prevent or restrict the transmission of data using intensity modulation at certain frequencies. An adaptive multi-carrier modulation transmitter may characterize an existing fiber optic channel and ascertain the overall characteristics of the channel. The transmitter and receiver can then be configured to use various bandwidths and various modulations in order to match the transfer characteristic of the fiber channel. A series of adaptive multi-carrier modulation transmitters and receivers can be integrated on a single integrated circuit. If multiple adaptive receivers and transmitters are integrated on a single integrated circuit, they may be used to upgrade existing networks by adding different wavelength lasers for the transmission of data in order to achieve any capacity desired. Each receiver and transmitter may characterize the fiber for its particular wavelength laser and may configure the modulation and bandpass to the fiber&#39;s characteristics.

PRIORITY

This application claims the benefit from provisional application60/160,501 filed on Oct. 20, 1999.

INCORPORATION BY REFERENCE

This application incorporates the content of the application “FULLYINTEGRATED TUNER ARCHITECTURE” filed on Nov. 12, 1999, Ser. No.09/439,101.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for the transferof high rates of data over a fiber optic channels, and in particularembodiments to methods and apparatus which utilize existing fiber opticssystems to achieve high data transfer rates.

BACKGROUND OF THE INVENTION

The amount of data carried on fiber optic systems continues to increase.The IEEE (Institute of Electrical and Electronics Engineers) 802.3aetask force is currently working on the definition of a 10 gigabit persecond standard for Ethernet applications. Although optical transmissionat 10 gigabits per second is possible with current technology, the priceof obtaining a 10 gigabit per second data rate may currently be veryhigh because of the necessity of using costly optical components.

Inexpensive optical components, such as fibers and lasers, may result inoptical channels with limited bandwith, nulls, significant noise,distortion and multi-mode transmission characteristics. Thesecharacteristics may be problematical when attempting to achieve highdata rates. Additionally, some of the fiber systems already in placecomprise fibers and components of lower quality than are currentlyavailable in modern fiber optic systems. The characteristics of thesesystems may also be problematical when attempting to increase thetransmission rates over such systems. There is therefore a need in theart to improve transmission capability of lower quality fiber opticsystems through the use of inexpensive electronics. There is also a needfor the use of channel coding and bandwidth efficient modulationtechniques to overcome the limitations of low quality optical channelsand result in higher transmission rates and reduced system costs.

SUMMARY OF THE INVENTION

Apparatuses for transmitting and receiving data on a fiber channel aredisclosed. An example transmitting apparatus comprises an input thatreceives a digital signal to be transmitted. The apparatus alsocomprises a plurality of programmable modulators each configured toaccept a portion of the digital signal to be transmitted, and tomodulate the portion of the digital signal accepted. The modulatorscommunicate with a plurality of mixers, each mixer coupled to the outputof one of the programmable modulators to accept a modulated signal andmix it with a mixer frequency. The mixed frequencies are then coupledinto a plurality of lowpass filters that filter the output of themixers. A summation unit then combines the output of the mixers into asingle signal.

An example receiving apparatus comprises an input that receives atransmitted optical signal and converts it into an electrical signal.The receiving apparatus also comprises a plurality of programmablemixers which accept the received electrical signal. The mixers arecoupled to programmable frequency sources. The mixed frequencies arethen coupled into a plurality of bandpass filters that filter the outputof the mixers. The filtered signals are then provided to programmabledemodulators, which will demodulate the signal according to themodulation used by the transmit apparatus. The demodulated signals arethen provided to a XGMII (Ten Gigabit Media Independent Interface),which combines the demodulated signals into a parallel bit stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions refer to drawings in which consistent numbersrefer to like elements throughout.

FIG. 1A is a graphical illustration showing a light pulse transmittedthrough a multimode fiber optic channel and resulting received pulses.

FIG. 1B is a graph of an exemplary frequency response of the opticalchannel exhibiting nulls.

FIG. 1C is a graph illustrating the impulse response of a multimodefiber channel in an “overfilled launch conditions.”

FIG. 1D is a block diagram of an exemplary fiber optic data transmissionsystem.

FIG. 2A is a block diagram of a multi-carrier modulation (MCM)transmitter.

FIG. 2B is a block diagram of a digital implementation of a QAM-16modulator, such as illustrated in FIG. 2A.

FIG. 2C is a block diagram of an analog implementation of a QAM-16modulator, such as illustrated in FIG. 2A.

FIG. 3 is a block diagram illustrating a multi-carrier modulationreceiver.

FIG. 4 is a block diagram of an alternate implementation of amulti-carrier modulation transmitter.

FIG. 5 is a block diagram of an alternate implementation ofmulti-carrier modulation receiver as may be used the alternativeimplementation of the MCM transmitter illustrated in FIG. 4.

FIG. 6 is a graph of intensity vs. frequency representing the transferfunction of an exemplary fiber optic channel.

FIG. 7A is a block diagram of a circuit implementation as may be usedwith fiber channels exhibiting nulls and/or noisy bands.

FIG. 7B is a flow diagram illustrating the process by which an existingfiber channel may be characterized.

FIG. 8 is a block diagram of a receiver as may be used with thetransmitter illustrated in FIG. 7A.

FIG. 9 is a block diagram illustrating the use of a set of multi-carriermodulation transmitters and multi-carrier modulation receivers inparallel in order to increase the data-carrying capacity of a channel.

FIG. 10 is a block diagram illustrating a modification of thetransmitter of FIG. 2A to incorporate trellis coding.

FIG. 11 is a block diagram illustrating a modification of the alternatetransmitter of FIG. 4 to incorporate trellis coding.

FIG. 12 is a block diagram illustrating a modification of the receiverof FIG. 3 to receive a trellis coded signal.

FIG. 13 is a block diagram illustrating a modification of the receiverof FIG. 4 to receive a trellis coded signal.

DETAILED DESCRIPTION OF THE INVENTION

Optical communications systems may achieve very high data rates as aresult of the high bandwidth of the optical fibers and the availabilityof high-speed lasers and photo detectors, as well as wideband laserdrivers, transimpedance amplifiers, postamplifiers, and clock and datarecovery circuits. However there is a demand for ever increasing speedin communications, and technology is rapidly approaching a point wherethe optical fiber can no longer be treated as having unlimitedbandwidth. This is particularly true for the case of multimode fibers,which may exhibit lower data carrying capacity due to the multimodenature of the fiber. Some modern optical communications systems may belimited in their speed by the bandwidth of the optical fibers.

Bandwidth efficient communications systems have been developed overseveral decades in the context of narrowband communications, such asvoiceband modems and digital subscriber loops. These systems make use oftechniques such as multilevel Pulse Amplitude Modulation (PAM) orQuadrature Amplitude Modulation (QAM) to encode several bits pertransmitted symbol, thus increasing the effective transmitted data ratewithout increasing the symbol rate or the required bandwidth of thecommunication channel. To make the most efficient use possible of theavailable channel bandwidth, channel equalization techniques may also beused to compensate pulse dispersion and the resulting inter-symbolinterference caused by the frequency-dependent attenuation and phasedistortion introduced by the channel. Channel coding and codedmodulation further increases bandwidth efficiency by allowing thecommunication system to operate at lower signal to noise ratios. For afixed amount of noise in the channel, the ability to operate at reducedsignal to noise ratios implies that the number of bits encoded in eachsymbol can be increased, resulting in higher bandwidth efficiency. Theuse of bandwidth efficient modulation techniques has resulted inpractical communication systems that approach the Shannon bound forchannel capacity. However so far these bandwidth efficientcommunications systems have been implemented at relatively low datarates, since they require complex signal processing techniques that arevery difficult to implement at high speed.

In general bandwidth efficient modulation techniques have not beenapplied to fiber optic systems. This is so partly because the opticalfibers provide so much bandwidth that, in many applications, it isunnecessary to use bandwidth efficient modulation techniques.Additionally, it has been difficult to implement bandwidth efficientmodulation techniques at the data rates normally used in opticalcommunications systems. Accordingly many optical communication systemshave been based, with a few exceptions, on simple bandwidth inefficientmodulation techniques. With the increasing demand for high speedcommunications systems, however, as the bandwidth demands increase,optical communications are reaching a point where bandwidth efficientmodulation techniques will be needed. Thus the problem of how toimplement complex digital signal processing (DSP) and coding algorithmsat high speed needs to be solved. Furthermore, a highly cost effectiveway to implement these complex algorithms is in a single monolithic chipor a chip set consisting of a small number of chips, for example in CMOS(Complementary Metal Oxide Semiconductor) technology. In order to reducecost, it is desirable to reduce as much as possible the complexity ofthe optical processing, even at the expense of greatly increasing thecomplexity of the electronic and DSP functions. As an example, it isdesirable whenever possible, to replace wavelength division multiplexing(WDM) techniques, which commonly incorporate several laser lightsources, by modulation, coding, and signal processing techniques thatprovide the same data rate over a single optical carrier. By utilizing asingle optical carrier the need for optical multiplexers anddemultiplexers, as well as the plurality of lasers and photodetectorsinherent in WDM techniques may be eliminated.

It is also desirable to replace optical equalization techniques byDSP-based equalization techniques, in order to take advantage ofelectronic components which are generally less expensive than theiroptical counterparts. Furthermore, it is desirable to achieve highlevels of chip integration, which requires that all analog processingfunctions, such as amplifiers, mixers, analog filters and dataconverters be integrated in a single chip or a small chip set. The useof techniques that lend themselves well to VLSI (Very Large ScaleIntegration), is highly desirable.

Commonly data rates of optical communications systems may exceed themaximum clock speeds of the digital signal processors that can beimplemented in current VLSI technologies. Accordingly to process thehigher data rates available in optical fibers parallel processingarchitectures may be employed. In general, parallel processingarchitectures reduce the required processing speed by partitioningcomputations into a set of subcomputations. The subcomputations may thenbe executed in parallel by assigning a separate processor to eachsubcomputation. Accordingly, the implementation of high-speed algorithmsin a transceiver may be accomplished by an array of processors runningat reduced clock speed executing tasks in parallel. To exploit theparallelism of an array of processors signal processing algorithmsshould be of such a nature that they may be partitioned into a number ofsubcomputations to be executed by the parallel processors present in thearray.

An important consideration in parallizing the computations is that thecomputational load of the different processors be balanced, in order toavoid bottlenecks that may result when one processor has a significantlyincreased processing load as compared to the other processors in thearray. Not all signal processing algorithms lend themselves well to aparallel processing implementation, therefore the choice of modulation,equalization and coding algorithms must be made with the requirement ofparallel processing implementation in mind.

One aspect of the present disclosure is that it may provide bandwidthefficient modulation techniques that can be applied to opticalcommunications channels in general. These disclosed techniques may befound to be particularly advantageous in channels whose bandwidth islimited, for example as a result of the use of multimode fibers.

A further aspect of the present disclosure is in providing techniquesthat enable the implementation of these bandwidth efficient modulationalgorithms at high speed and in the form of a single monolithic chip ora chip set consisting of a small number of chips, for example in CMOStechnology.

The present disclosure also describes modulation, equalization, coding,and data conversion algorithms and architectures that lend themselveswell to parallel processing implementations which may enable thereducing the clock speed of VLSI implementations. Further useful aspectsof this disclosure will become apparent upon reading and understandingthe present specification. Optical channels based on intensitymodulation and direct detection are inherently nonlinear. This isbecause the modulated quantity is the optical power rather than theelectromagnetic field. The principle of superposition applies to theelectromagnetic field, as a result of the linear nature of Maxwell'sequations. Superposition does not directly apply to the optical power.However, it has been shown that under certain very general conditions,the intensity-modulated optical channel behaves approximately linearly,and the principle of superposition can be applied. This linearapproximation of an optical channel is commonly referred to as the“quasi-linear approximation”. The conditions under which the“quasi-linear approximation” is valid have been analyzed in the article“Baseband Linearity and Equalization in Fiber Optic DigitalCommunication Systems”, by S. D. Personick, appearing in the Bell SystemTechnical Journal, Vol. 52, No. 7, September 1973, pages 1175-1194.Nonlinear distortion may occur in an optical channel when the conditionsfor the validity of the quasi-linear approximation do not hold.Furthermore, nonlinearities in the transmitter coupled to the fiberchannel (such as laser or drive electronics non-linearities) mayintroduce an additional source of nonlinearity even if the quasi-linearapproximation holds.

In the following discussion we assume that the quasi-linearapproximation holds, and treat the optical channel as linear.

Another impairment that can exist in an optical channel is dispersion.This is a linear effect. There are different sources of dispersion inoptical fibers. One type of dispersion is chromatic dispersion, causedby the dependence of the index of refraction on the wavelength of thelight. If the laser is not perfectly monochromatic (as commonly happensin practice), light components of different wavelengths travel atdifferent speeds, causing dispersion of the transmitted pulses. Commonlychromatic dispersion is small and it causes problems only in long hauloptical links. Another type of dispersion is the multimode dispersion,which exists in multimode fibers. The source of this type of dispersionis that different optical fiber modes propagate at different velocities.A transmitted pulse splits its energy among many different modes. Sincethe light pulse carried by each mode travels at different speed, thereceiver observes multiple replicas of the transmitted pulse, arrivingat different times, as shown in FIG. 1A.

FIG. 1A is a graphical illustration showing a light pulse transmittedthrough a multimode fiber optic channel and the resulting receivedpulses. Pulse 101 is transmitted. Because pulse 101 travels throughmultiple modes in the fiber at different speeds pulses 103, 105, 107,109 and 111 are received at the receiver. If the fiber attenuation isvery small and can be neglected, the sum of the energies of all thereceived pulses equals the energy of the transmitted pulse. In practice,since the fiber has a nonzero attenuation, the sum of the energies ofall received pulses will be less than the energy of the transmittedpulse.

As a simplified example, suppose that the transmitted pulse splits itsenergy equally into only two modes. Also, suppose that, as a result ofthe different propagation velocities of the two modes, these pulsesarrive at the receiver with a time separation T, and they suffer zeroloss. Then if the energy of the transmitted pulse is 1 (in arbitraryunits) the two received pulses have energy 0.5 (in the same arbitraryunits). Neglecting the propagation delay of the line, the impulseresponse of the fiber is:

0.5 δ(t)+0.5 δ(t−T)   Equation (1)

where δ(t) is the Dirac delta function. The Fourier transform of thisimpulse response is the frequency response of the optical channel, andit is given by:

H(ω))=exp(−jωT/2)cos(ωT/2)   Equation (2)

FIG. 1B is a graph of the magnitude of the Equation (2) function. It isclear that this frequency response exhibits nulls at all odd multiplesof (½ T). This example illustrates how multimode transmission can giverise to nulls in the frequency response of the fiber. In practicemultimode fiber propagates many modes, typically more than one thousand.This multimode propagation may result in rather complex impulse andfrequency responses. The impulse and frequency responses are alsodependent on the way the laser light is coupled to the fiber. If allmodes of the fiber are excited equally, it is said that the channel isoperating in “overfilled launch” conditions. This results in an impulseresponse that closely approximates a Gaussian function, as shown in FIG.1C, which is a graph illustrating the response of a multimode fiberchannel to an “overfilled launch.”

The analytic expression for the impulse response illustrated in FIG. 1 Cis:

h(t)=1/((√{square root over (2π)})αT)exp(−t ²/2(αT)²)   Eqn. (3)

Where α is a fiber dependant parameter. The corresponding frequencyresponse is given by:

H(f)=exp(−(2παTf)²/2)   Equation (4)

H(f) is also a Gaussian function, so in the case of overfilled launchconditions, the frequency response does not exhibit nulls. The 3 dBoptical bandwidth is:

B=0.1874/(αT)   Equation (5)

This is called the “overfilled bandwidth” of the multimode fiber. Theoverfilled bandwidth of a fiber depends on the core diameter andrefraction index profile of the fiber as a function of the radius, thefiber length, and the wavelength of the laser, among other things. Thebandwidth is inversely proportional to the length of the fiber, so thatthe product of the bandwidth times length is a constant for a given typeof fiber and a given laser wavelength. For example, typical fibers inwidespread use today have an overfilled bandwidth of 500 MHz·Km at awavelength of 1310 nm, and 160 MHz·Km at a wavelength of 850 nm.

A number of alternative launch techniques such as offset launch andvortex launch have been developed in order to excite a reduced number offiber modes. These techniques could result in increased bandwidth, butthe impulse response could also show multiple peaks, as in the exampleof FIG. 1A. This kind of impulse response usually results in a frequencyresponse with nulls similar to the ones shown in FIG. 1B, but with amore complicated structure. The exact shape of the frequency and impulseresponses is a complicated function of the fiber characteristics and thelaunch conditions.

Various methods of transferring data over a fiber optic channel areknown in the art. Multi-carrier modulation or MCM is one method ofcommunicating over a fiber optic channel. In MCM, multiple carriers atdifferent frequencies are modulated with data. The modulated carriersare then summed and the sum signal is used to modulate a laser, forexample, by modulating the intensity of the laser beam.

Multi-carrier modulation systems may also be adapted for use in legacyfiber channels, which may have nulls and noisy frequency bands.Multi-channel modulators, according to embodiments of the presentinvention may have variable modulations and carrier frequencies. Thevariable modulations and carrier frequencies of MCM may be selectivelychosen in order to accommodate nulls, noisy frequency bands, and othertransmission impediments of existing and/or low quality channels. Nullsand noisy frequency bands can be determined in an existing channel, forexample, by introducing a test signal and measuring the received result.Multi-channel modulators may also be used, in parallel, in order toupgrade existing fiber channels to virtually any capacity desired. Toupgrade the capacity of existing fiber channels, multiple MCMs may beadded in parallel using multiple lasers having different wavelengths.Because each laser may have its own response within the channel, eachchannel may undergo an initialization, in which a test signal is used todetermine the frequency characteristics of the channel at thatparticular laser wavelength.

Existing fiber channels may be upgraded into any data-carrying capacitydesired by simply adding more MCMs and multiple lasers, having differentwavelengths, in parallel until the desired data-carrying capacity isachieved.

FIG. 1D is a block diagram of an exemplary fiber optic data transmissionsystem. Digital data is coupled into a data processing block 101. Dataprocessing block 121 processes the input data and prepares it for theoptical transmitter 123. The optical transmitter 123 transmits the dataacross a fiber optic channel 125. The optical receiver 107 accepts thedata from the fiber optic channel 125 and then couples it into a dataprocessing module 129. The data from the optical receiver 107 isprocessed in the data processing module 129 and then provided to theapplication, which will receive data.

The blocks within the block diagram of FIG. 1D may have a variety ofpossible implementations. For example, in a bit serial approach, thedata processing block 121 may accept data and convert the data into aserial bit stream. The optical transmitter 123 may then accept theserial bit stream from the data processing block 121 and transmit as aserial type on/off keying (OOK) signal. In on/off keying a laser withinthe optical transmitter block 123 is ideally turned on and off dependingon whether the data that is received from the data processing block 121is a one or zero.

In practice, the laser is never turned off completely, because theturn-on time of typical lasers is relatively long, therefore therepeated turn-on transients would degrade the speed of the opticaltransmission system. Instead the optical power is switched between ahigh value (representing a discrete logic level such as a “1”) and a lowvalue (representing a discrete logic level such as a “0”). The ratio ofthe high to low optical power is commonly called the “extinction ratio.”The laser may also be switched between several intensity levels in orderto represent symbols from a logical alphabet.

For the purposes of simplifying this disclosure when two levels ofoptical power are transmitted the high and low levels may be referred toas on and off optical pulses, but it should be understood that an “off”pulse will typically carry a non zero amount of optical power. In anexemplary fiber optic system according to FIG. 1, data is conveyed as aseries of on/off optical pulses from the optical transmitter 123 throughthe fiber 125 to the optical receiver 127. The optical receiver 127 willthen provide the on/off data stream to the data processing module 129.The data processing module will then convert the serial on/off stream toa data output, for example, in a parallel type format.

The blocks in FIG. 1D may also be configured in a Wavelength DivisionMultiplexing (WDM) approach. Wavelength division multiplexing approachesare, in general, divided into dense and coarse WDM, commonly referred toas DWDM and CWDM respectively. The wavelength division multiplexingtechnique employs multiple optical carriers, which are modulated by adata stream. The dense version of wavelength division multiplexinggenerally comprises laser carriers whose wavelengths differ by less thanone nanometer. Coarse wavelength division multiplexing generallycomprises laser carriers whose wavelengths differ by ten nanometers ormore. Dense wavelength division multiplexing systems have been createdwith, for example, up to 100 different wavelengths. Such systems areexpensive. A major contributing factor to the expense of wavelengthdivision multiplexing systems is the multiple optical sources for thedifferent wavelengths, which carry the data from the transmitter to thereceiver. Additionally the expense of combining the multiple opticalcarriers, in WDM systems, adds to their cost. Additionally, once thedata carriers arrive at the optical receiver 127 they commonly must thenbe optically split, in order to retrieve the data streams from theindividual optical carriers.

It is desirable, from a cost standpoint, to employ a single opticaltransmitter 123 and a single optic receiver 127 if possible. It is alsodesirable that the optical transmitter 123 and optical receiver 127 beas simple as possible and any processing accomplished in the electronicdata processing blocks 121 and 129. In a preferred embodiment of thepresent invention, data processing block 121 as well as data processingblock 129 comprise a single integrated circuit each. By transferring allfunctions except those used for the actual optical transmission into,for example, a CMOS (Complementary Metal Oxide Semiconductor) integratedcircuit, a low cost implementation of the transmit side of the system inFIG. 1 can be realized. Likewise, if the optical receiver 127 isrestricted to those components needed for optical reception and allother functions are performed within the data processing block 129, (forexample a CMOS integrated circuit), the costs for the received portionof this circuitry of FIG. 1 can be minimized. Once such embodiment of amulti-carrier modulation transmitter, which can be integrated into asingle CMOS circuit, is illustrated in FIG. 2.

FIG. 2A is a block diagram of a multi-carrier modulation transmitter.The illustrative MCM transmitter of FIG. 2A has a capacity of 10gigabits/second. In FIG. 2, 64 bits of data 201 are coupled into a XGMII(Ten Gigabit Media Independent Interface) module 203. The XGMII is shownas providing a 64 bit input or output at a clock rate of 156.25 MHz. Inreality the XGMII interface operates on 32 bits at a time, and isclocked on both the rising and the falling edges of the 156.25 MHzclock. A complete description of the XGMII interface is given in thepresentation “10 Gig MII Update” by H. Frazier, presented at thee IEEE802.3 High Speed Study Group Plenary meeting in Kauai, Hi, on Nov. 9,1999. A copy of this presentation can be obtained from the IEEE 802.3web site, at:

-   -   http://grouper.ieee.org/groups/802/3/10G_study/public/nov99/frazier_(—)1_(—)1199.pdf.

Other system interfaces besides the XGMII have been under considerationby the IEEE802.3ae Task Force, and will likely be included in thestandard. One important alternative interface is the XAUI interface,described in the document:

-   -   http://grouper.ieee.org/groups/802/3/ae/public/may00/taborek_(—)2_(—)0500.pdf        Those skilled in the art will recognize that the interfaces        illustrated within this disclosure are by way of example and are        chosen as examples likely to be familiar to those skilled in the        art. Other interfaces may also be used. The uses of the        exemplary interfaces within this disclosure are in no way meant        to limit the inventive concepts to those interfaces and serve        the purpose of illustration and example only.

The 64 bits are then divided into four bit nibbles and further providedto the modulators. Modulator zero, 205 is a QAM-16 (Quadrature AmplitudeModulation) module. The QAM-16 module 205 accepts four bits of data fromthe XGMII module 203. The frequency that is mixed with the output of theQAM-16 modulator in mixer 207 is zero megahertz. After the mixing stage207, the signal is bandpass filtered in block 209 to remove anyundesirable components. The filtered signal is then coupled into acombiner 211. Similarly, the second four bits from the module 203 arecoupled into QAM modulator 213. The output of the QAM-16 modulator 213is coupled into a mixer 215 where it is mixed with a 200 megahertzsignal. The resultant signal is then bandpass filtered, in bandpassfilter 217, and coupled into the combiner 211. The signals which arecoupled into the combiner 211 from bandpass filter 217, occupies afrequency bandwidth from 200 megahertz to 356.25 megahertz. Each mixerwithin the exemplary multi-carrier modulation block diagram illustratedin FIG. 2 has a frequency which is separated from successive mixerfrequency by 200 megahertz. In such a way by having the bandpass lessthan the separation of mixing frequencies may minimize the crosstalkbetween the modulators.

In the final stage, four bits are coupled from the XGMII modulator 203into the final QAM-16 modulator 219. The frequency of modulation, whichis coupled into mixer 221, is 3.0 gigahertz. The output of mixer 221 isthen coupled into bandpass filter 223 to remove any unwanted components.The filtered signal is then coupled from the bandpass filter 223 intothe combiner 211. The combiner 211 combines all the modulated frequencybands into a single frequency spectrum. The frequency spectrum signal isthen further coupled to laser drive electronics 213. The output of thelaser drive electronics 213, is an intensity modulated signalcorresponding to the combination of all 16 modulators. All of theelectronics illustrated in FIG. 2 and encompassed within the dotted line225 may be integrated into a single integrated circuit, for example aCMOS integrated circuit chip. By integrating all the electronics withinthe block 225 on a single chip, the system cost may be reduced. Byplacing as much of the processing as possible within the integratedcircuit 225, costs of assembly, alignment and multiple optical parts maybe greatly reduced.

FIG. 2B is a block diagram of a digital implementation of a QAM-16modulator; such as illustrated in FIG. 2A. The QAM 16 Mapping block 229accepts a four bit input signal at a frequency of 156.25 MHz. The QAM 16Mapping block 229 then maps the four input bits into I (inphase) and Q(quadrature) representations of the input bits received The I and Qvalues are then provided to Square Root Raised Cosine Filters 231 and233 respectively. The sampling frequency f_(S) of Square Root RaisedCosine Filters 231 and 233 is typically three times the symbol rate orhigher (in the present example 468.75 Mhz or higher). Because of the DSPbased implementation, the carrier frequency ω₀=2πf₀ must be limited to arelatively low value such as 100 Mhz. Higher carrier frequencies wouldrequire an increase in the sampling frequency f_(S) of the square rootraised cosine filters and D/A Converters making the implementationdifficult. The signals are then multiplied in blocks 235 and 237 andsummed in block 239. The summed value is then coupled into a Digital toAnalog converter 241, and then further into a smoothing filter prior tothe coupling of the signal to the mixer (e.g. 207).

FIG. 2C is a block diagram of an analog implementation of a QAM-16modulator, such as illustrated in FIG. 2A. The QAM 16 Mapping block 229accepts a four bit input signal at a frequency of 156.25 MHz. The QAM 16Mapping block 229 then maps the four input bits into I (inphase) and Q(quadrature) representations of the input bits received The I and Qvalues are then provided to Square Root Raised Cosine Filters 231 and233 respectively. The outputs of the Square Root Raised Cosine Filters231 and 233 are then coupled into D/A converters 245 and 247, thenfurther provided to smoothing filters 249 and 251, prior to being mixedin mixers 253 and 255, and then further combined in 257. In thisapproach the modulation can be done at the carrier frequency directlyf_(C) directly, avoiding the need of the mixer and bandpass filter.However, this approach requires that the amplitude and phase of the Iand Q carriers cos(ωct) and sin(ωct) be very accurate.

FIG. 3 is a block diagram of a MCM receiver. The signal from the laserdrive electronics 213 is provided to a laser, for example a solid statelaser. The output of the laser is coupled to a fiber optic channel. Theoutput of the fiber optic channel is provided to a photo detector andamplifier 301. The photo detector and amplifier 301 converts the opticalsignal into a voltage level. The photo detector and amplifier 301 thencouples the amplitude modulated signal into a series of mixers. Theoutput of the photo detector and amplifier 301 corresponds to the outputof the combining circuit 211. The first mixer 303 mixes the incomingsignal with zero megahertz. In other words, the signal is basicallycoupled into a low pass filter 305. A mixer 303 is illustrated eventhough no mixing is occurring because, in other embodiments of the MCMsystem, a first mixer, such as 303, may provide a programmable mixingfunction. Low pass filter 305 filters the incoming signal and provides abaseband frequency signal to a QAM-16 demodulator 307. The QAM-16demodulator demodulates the baseband signal provided to it by the lowpass filter 305. The output of the QAM 16 demodulator is a four bit datasignal which is then provided to the XGMII 309. Similarly, the secondmixer 313 accepts the amplitude modulated signal from the photo detectorand amplifier 301. The signal is then mixed with a 200 megahertz mixersignal which translates a portion of the signal bandpass, correspondingto 200 to 400 megahertz, into a baseband signal. The baseband signal isfiltered in the low pass filter 315 thereby eliminating other unwanted,non-baseband signals. The output of the low pass filter is then coupledinto a QAM-16 demodulator 317. The output of the QAM-16 demodulator 317is a four bit symbol which is coupled into the XGMII 309.

Additional mixers (e.g. 303 and 313) are provided with mixing signalsseparated by 200 megahertz. The 16^(th) mixer 319 accepts the bandpasssignal from the photo detector and amplifier 301, and mixes it with athree gigahertz signal. The output of the mixer 319 is then coupled intoa low pass filter 321. The low pass filter 321 passes the resultingbaseband signal and provides it to the QAM-16 demodulator 322. TheQAM-16 demodulator demodulates the signal into a four bit data signal.The four bit symbol is then coupled into the XGMII (Ten Gigabit MediaIndependent Interface) unit 309. The XGMII combines all 16 input symbolsprovided to it to produces a stream 311 64 bits wide.

FIG. 4 is a block diagram of an alternative implementation of amulti-carrier transmitter. In FIG. 4, a 64 bit wide digital signal 401is coupled into a XGMII 403. The XGMII interface separates the bits intofour bit groups. Each four bit group is coupled into an assemblingcoder, for example, 405, 415 or 419. The output of each symbol encoderis then coupled into an inverse fast Fourier transform block 407.Digital to analog converters for example, 409, 417 and 421 convert theoutput of the inverse fast Fourier transform (IFFT) block 407 intoanalog samples. The analog samples are then coupled into an analogmultiplexer 411 with a sampling frequency of five gigahertz. The analogmultiplexer multiplexes all analog signals into a single waveformcomprising successive samples. The analog waveform is then coupled intolaser drive electronics 413 and is used to modulate the laser intensityusing drive electronics 413. The implementation illustrated in FIG. 4may be integrated into one integrated circuit device 423 capable ofaccepting a 64 bit wide digital bit stream. In the present embodiment,the 64 bit wide digital signal stream has a frequency of 156.25megahertz resulting in an overall data carrying capacity of 10 gigabitsper second. The laser drive electronics 413 intensity modulates thelaser beam, which is then coupled into a fiber optic channel. The otherend of the fiber optic channel is then coupled into the photo detectorand amplifier 501 of the receiver module, for example, as illustrated inFIG. 5.

FIG. 5 is a block diagram of an alternate implementation ofmulti-carrier modulation receiver, corresponding to the alternativeimplementation of multi-carrier modulation transmitter illustrated inFIG. 4. The photo detector and amplifier 501 receive the multi-carriermodulated signal from the fiber optic and convert it into an electricalsignal. The resulting electrical signal from photo detector andamplifier 501 is coupled into an array of sample and hold circuits, forexample sample and holds 503, 515 and 521. The sample and hold circuitsamples successive values in the input waveform. The output of thesample and hold circuits are then coupled into analog to digitalconverters such as for example 505, 517 and 523. The digital outputsfrom the analog to digital converters are further coupled into a fastFourier transform module 507. The sequence of symbols which comprise theoutput of the fast Fourier transform block 507 are coupled into symboldecoders, for example, 509, 519 and 525. The output of each symboldecoder is a set of four data bits. The set, of four bits from eachsymbol decoder is coupled into the XGMII interface 511. The XGMIIinterface 511 receives the four bit input from the symbol decoders andforms a 64 bit parallel output. The 64 bit parallel output from theXGMII 511 is clocked out at a frequency of 156.25 megahertz. The outputof the XGMII therefore represents a 10 gigabit per second digital datastream. The entire receiver represented by the block 525 can beintegrated within a single integrated circuit, such as a CMOS integratedcircuit.

The transmission techniques illustrated in FIGS. 2, 3, 4 and 5 canprovide for 10 gigabits per second data transmission at a symbol rate of156.25 megahertz by utilizing the intrinsic parallelism present in thesub-carrier multiplexing technique.

If a non-parallel method were used to transmit 10 gigabits per second ata symbol rate of 156.25 megahertz, 64 bits per symbol would be requiredto achieve the 10 gigabit per second data rate. This would result in amodulation constellation of astronomical complexity and low noiseimmunity. Even if the symbol rate was quadrupled to 625 megahertz, a65,536 QAM constellation would be required. In addition, because the156.25 megahertz rate is relatively low, multi-carrier modulation isrobust and multi-path propagation effects, such as multi-mode dispersionin fibers, is minimized due to the low transmission frequency. Thoseskilled in the art will recognize that the multi-carrier modulationtechnique just described is exemplary. Many variations of individualbandpasses, modulation constellations and carrier frequencies arepossible within the scope of the inventive concepts of this disclosure.

The multi-carrier modulation concepts illustrated in FIGS. 2, 3, 4, and5 may also be used to increase the data rate within current deployedfiber networks. A difficulty that may be encountered with current fibernetworks is that some of the fibers used in current fiber channels maybe of lesser quality.

FIG. 10 shows a modification of the transmitter of FIG. 2A thatincorporates trellis coding. The 4-bit input from the XGMII block ispassed to a rate 5/4 convolutional coder. The 5-bit Output from theconvolutional coder is modulated using a QAM-32 modulator. Trelliscoding requires an expanded constellation, but does not increase thesymbol rate of the QAM modulator or the bandwidth of the modulatedsignal. The same idea can be applied to the alternative transmitterimplementation of FIG. 4. This is shown in FIG. 11. The modifiedreceivers incorporating trellis decoders for the implementations ofFIGS. 3 and 5 are shown in FIGS. 12 and 13 respectively. The use oftrellis coding results in increased robustness and decreased error ratein the presence of noise and other channel impairments. While FIGS. 10through 13 show particular embodiments of trellis coding and decodingfor an optical communications transceiver, it will be clear to anyoneskilled in the art that many alternative embodiments are possiblewithout departing from the spirit and scope of the present invention.

Those fiber channels may exhibit channel anomalies such as deep nulls,particularly in the case of fibers with multi-mode dispersion. Suchnulls are illustrated in FIG. 6.

FIG. 6 is a graph of intensity versus frequency representing thetransfer function of an exemplary fiber optic system. The ideal responseis linear, that is the transmitted intensity does not vary as the laserlight output is modulated at different frequencies. However fiber opticsystems, particularly those exhibiting multi-mode interference, mayexhibit nulls such as 603 and 605. The nulls 603 and 605 may be due inpart to a large number of different factors such as multi-modedispersion within the fiber, imperfect coupling of the fiber to thelight source, frequency response of driver electronics and a variety ofother factors.

FIG. 7A is a block diagram of a circuit implementation, as may be usedwith fiber channels exhibiting nulls, noise and/or frequency roll off.In FIG. 7, a parallel bit stream 701 comprising bits 0 through 63 iscoupled into a XGMII interface. The XGMII interface is controlled by amicro controller 727 through a controlling bus 729, such as an I²C bus.The control bus 729 controls the XGMII controller so that the number ofbits which are provided to each modulator such as 705, 713 and 719, arevariable. Additionally, the micro controller 727 using the control bus729 may control the modulation constellations of the modulators withinthe system. The control bus may also control the modulation frequenciessuch as 731, 733 and 735 which are applied to mixer 707, 715 and 721,respectively. The micro controller 727 may also control the bandpassfilters such as 709, 717 and 723. The micro controller 727 may controlthe system variables so that the mapping of data from the input 701through the modulators and frequency bands avoids fiber channelanomalies such as nulls within the optic system, for example the nullsillustrated at 603 and 605 of FIG. 6.

In order to configure the circuit 225 to avoid nulls and to match thenoise response of the channel the configuration of the transmitter canbe arranged to match the transfer characteristics of the fiber channel.To arrange the configuration of the transmitter the transfercharacteristics of, the fiber channel characteristics need to bedetermined. Such a process to determine the frequency response of thefiber channel is illustrated in FIG. 7B.

FIG. 7B is a flow diagram illustrating the process by which the transfercharacteristics of a fiber channel may be mapped. The process begins inblock number 741 when the transmitter and receiver are coupled to afiber channel. In block 743, the transmitter then transmits an intensitymodulated signal, preferably having a flat frequency characteristic, inwhich the frequency of intensity is swept from the minimum frequency, tobe used on the channel, to the maximum frequency, to be used on thechannel. In block 745, the receiver receives the signal and measuresintensity versus frequency. This measurement of intensity versusfrequency between the frequencies of F_(min) and F_(max) willcharacterize the frequency transfer characteristic of the fiber channel.The measurements of intensity versus frequency are then communicatedfrom the receiver to the transmitter. In block 747 the transmitter mapsthe fiber channel response onto the circuitry of the transmitter. Theprocess then ends in block 749.

As an illustration of configuring the transmitter according to the fiberchannel response, FIG. 2A is compared with FIG. 7A. In FIG. 2, forexample, modulator 213 receives a four bit value. Modulator 213 utilizesa QAM-16 modulator. The carrier frequency is 100 megahertz and the bitrate is 156.25 megahertz. The bandpass filter 217 is then set to a valueof 0 to 200 megahertz. The other modulators and bandpass filters couldbe similarly set according to channel characteristics.

If for instance one half of the 200 megahertz baseband frequency of thebandpass filter 217 corresponded to a null within the fiber opticchannel, the null could be compensated for (as shown in FIG. 7A) byreducing the symbol rute rw modulator 713. Modulator 713 would then onlyaccept 2 bits of data from the XGMII interface 703 at a time. The microcontroller 727 could program the XGMII 703 to couple only 2 bits at atime into modulator 713. If on the other hand, the particular bandcorresponding to modulation frequency two were exceptionally free ofnoise, the XGMII could be controlled by the micro controller 727 tocouple five bits into modulator 713 and QAM-32 could be used withinmodulator 213. Similarly, the bandwidths of the carriers could beadjusted to be wider or narrower and carry more or less informationdepending on the characteristics of the fiber channel. The mapping, ofcourse, would have to be communicated either explicitly or implicitly tothe receiver in order for the data stream communicated to be properlydecoded. FIG. 8 is an example of a multi-carrier modulation receiverthat may be used with the transmitter illustrated in FIG. 7A.

FIG. 8 is a block diagram of a receiver as may be used with thetransmitter illustrated in FIG. 7A. In FIG. 8 the photo detector andamplifier 801 receives a signal from the fiber channel. The receiver 825has been configured to match the transmitter 725 in FIG. 7A. Thereceiver in FIG. 8 may be configured in a variety of ways. The receiverillustrated in FIG. 8 may be configured explicitly by receiving amapping of the channel transfer characteristic from the transmitter. Thereceiver in FIG. 8 may be also mapped implicitly by having the samemapping algorithm within the micro controller 827 as within the microcontroller 727. Because the mapping of the frequency characteristics ofthe fiber channel is accomplished by sending a signal from thetransmitter to the receiver and the receiver sending the characteristicsof the fiber channel to the transmitter, the receiver already knows thecharacteristics of the fiber channel because of the signal received fromthe transmitter. If the same algorithm for mapping modulation, mixerfrequencies and bandpass filter widths are used within the receiver aswithin the transmitter the configuration of the transmitter need not beexpressly communicated to the receiver only the overall response of thechannel need be communicated. For example, after the frequency responseof the channel dictated that 2 bits would be coupled into modulate 713at a time, the same algorithm within micro controller 827 wouldconfigure demodulator two block number 817 as an 8PSK demodulator.Alternately, the configuration from the transmitter circuit may becommunicated to the receiver circuit explicitly on start up. By usingthe transmitter in FIG. 7A, mapping the fiber channel and communicatingthe mapping to similarly configure a receiver as illustrated in FIG. 8,the capacity of any existing fiber channel, no matter thecharacteristic, can be accommodated. Additionally, the systemillustrated in FIG. 7A and FIG. 8 may be extended through the use ofmultiple wavelength lasers and parallel MCM transmitter receiver pairs.For example, an existing fiber can have two laser beams coupled throughit to communicate between a sending end and a receiving end. Such asystem is illustrated in FIG. 9.

FIG. 9 is a block diagram illustrating the use of a set of threemulti-carrier modulation transmitters and three correspondingmulti-carrier modulation receivers used in parallel in order to increasethe data carrying capacity of the illustrated fiber channel. In FIG. 9,three multi-carrier modulation transmitters 725A, 725B and 725C areemployed in order to intensity modulate the beams of three lasers 903,905 and 907. The embodiment of FIG. 9 is by way of illustration andexample only. The number of multi-carrier modulation transmitters,lasers and receivers that are used may vary from embodiment toembodiment depending on the capacity desired and the transfercharacteristics of the fiber channel 911 carrying the data. In FIG. 9,data is input serially to a splitter 901. Splitter 901 splits the dataso that three separate streams of data are coupled into threemulti-carrier modulation transmitter modules 725A, 725B and 725C. Themulti-carrier modulation transmitter modules 725A, 725B and 725C may beidentical modules. In a preferred embodiment, each module is implementedas a single integrated circuit chip, for example, a CMOS integratedcircuit chip. The multi-carrier modulation transmitters are furthercoupled into laser drive electronics 713A, 713B, and 713C, respectively.Laser drive electronic 713A drives laser 903, laser drive electronic713B drives laser 905, and laser drive electronic 713C drives laser 907.The outputs of lasers 903, 905 and 907 are combined optically in acombiner 909 which also couples the combined optical beams into a fiberoptic channel 911. The fiber optic channel 911 carriers the three laserbeams to a receiver and beam splitter 913. The receiver and beamsplitter 913 splits the output from the fiber optic channel 911 backinto its constituent laser carriers. The three laser carriers are thencoupled into photo detectors and amplifiers 801A, 801B, and 801C. Thephoto detectors and amplifiers convert the light signal into anelectrical signal which is then further coupled into multi-carriermodulation receivers 825A, 825B and 825C. Multi-carrier modules 825A,825B and 825C may be identical and may operate in parallel withoutneeding to be aware of the operation each other. The output of themulti-carrier modulation receivers are then combined in combiner block915A and produce an output data stream. Multi-carrier modulationtransmitters, lasers and multi-carrier modulation receivers can be addedas needed in order to achieve the capacity desired.

For example, if the fiber optic channel 911 is an existing fiber opticchannel for a local area network and it is desired to upgrade it toservice a 10 gigabit ETHERNET network, transmitters, lasers andreceivers may be added in parallel until the 10 gigabit capacity isachieved. The multi-carrier modulation transmitters and receiver pairsmay cooperate to characterize the fiber optic channel at each laserwavelength used. By combining multiple multi-carrier modulationtransmitters, as illustrated at 917, and multi-carrier modulationreceivers as illustrated at 919, a very efficient architecture forimplementing high speed data rate communications utilizing existencefibers can be accomplished. For example, by combining the data splitter901 with a plurality of multi-carrier modulation transceivers 725, asillustrated at 917, a number of lasers can be accommodated. Likewise, anumber of receivers 825 and a data combiner 915 can be combined in asingle integrated circuits 919. These circuits can then be coupled tolaser drive electronics and laser receiving electronics as needed inorder to achieve any capacity desired. Accordingly, existing fiber opticchannels 911 can be upgraded at reasonable costs without having toreplace existing fiber channels. The receivers and transmitters canadjust to the characteristics of the fiber channel and high speednetworks, for example, 10 gigabit communication networks, can beimplemented despite the fact even in fiber channels of inferior quality.

Even if fiber channels have grossly inferior transfer characteristicsthe channel may be upgraded to a high data rate by adding as many of theconfigurable multi-carrier modulation receivers in parallel as necessaryto achieve the desired data rates and noise immunities.

FIG. 10 is a block diagram illustrating a modification of thetransmitter of FIG. 2A to incorporate trellis coding.

Exemplarily a 4-bit input from the XGMII block 1003 is passed to a rate5/4 convolutional coder 1005. The 5-bit output from the convolutionalcoder is modulated using a QAM-32 modulator 1021 prior to being coupledto a mixer 1017. The mixer 1017 then couples the mixed signal to abandpass filter 1021, which filters out unwanted components and furtherprovides the filtered signal to a combiner circuit 1027. The combinercircuit 1027 combines all the bandpass signals provided to it by theplurality of bandpass filters and then provides the combined signal tothe laser drive electronics 1027.

Trellis coding requires an expanded constellation, but does not increasethe symbol rate of the QAM modulator or the bandwidth of the modulatedsignal, accordingly in the present exemplary embodiment a QAM 32modulator is used to accommodate a 5 bit input. Other modulators may beused depending on implementation requirements. The convolutional coderprovides a trellis encoding that enhances the robustness of the code andreduces such problems as inter-symbol interference.

FIG. 11 is a block diagram illustrating a modification of the alternatetransmitter of FIG. 4 to incorporate trellis coding. In FIG. 11 a 5/4convolutional coder accepts a 4 bit input and provides a 5 bit output.The addition of a convolutional encoder provides redundancy in the codeand provides a mechanism for reducing problems such as inter-symbolinterference.

FIG. 12 is a block diagram illustrating a modification of the receiverof FIG. 3 to receive a trellis coded signal. In FIG. 12 a QAM 32demodulator 1215 is disposed between the low pass filter (for example1209) and the XGMII 1227. The QAM 32 demodulator 1215 produces softdecisions for the purpose of providing them to a trellis decoder 1221.The demodulator 12 has been chosen as a QAM 32 demodulator in order tomatch the input signal. Other demodulators may be used corresponding tothe modulation of the transmit signal.

FIG. 13 is a block diagram illustrating a modification of the receiverof FIG. 4 to receive a trellis coded signal. In FIG. 13 the 32 InverseFourier Transform provides symbols (S) along with the complex conjugateof the signals (S*) to a trellis decoder. The trellis decoder decodesthe symbols and then provides a 4 bit output signal to the XGMII tocombine with other signals in order to produce the output signal.

The use of trellis coding results in increased robustness and decreasederror rate in the presence of noise and other channel impairments. WhileFIGS. 10 through 13 show particular embodiments of trellis coding anddecoding for an optical communications transceiver, it will be clear toanyone skilled in the art that many alternative embodiments are possiblewithout departing from the spirit and scope of the present invention.For example the convolutional coders may be changed, the modulators mayutilize other modulation constellations and modulation frequencies, andthe modulators, encoders, bandpass filters, demodulators, also may bechanged for example to accommodate the characteristics of the fiberchannel.

1-15. (canceled)
 16. A method for transmitting data on a fiber opticchannel, the method comprising: providing a test signal to a fiber opticchannel; receiving a characterization of the channel defined by achannel response to the test signal; programming a multicarriermodulator according to the characterization of the channel.
 17. A methodas in claim 16 wherein providing a test signal to a fiber optic channelfurther comprises providing an intensity modulated light source whereinthe intensity changes according to a frequency sweep signal, but themaximum and minimum intensity does not change.
 18. A method as in claim16 wherein programming a multicarrier modulator further comprisesselecting a type of modulation for a plurality of modulators, whereineach modulator corresponds to a modulation of one of a multicarriersignals.
 19. A method as in claim 16 wherein programming a multicarriermodulator further comprises providing a plurality of selected mixerfrequencies to a plurality of mixers, wherein the mixers determine thecarrier frequencies for the multicarrier modulator.
 20. A method as inclaim 19 further comprising programming a bandpass filter with a centerfrequency corresponding to the mixer frequency of the mixer whichprovides a signal to the bandpass filter.
 21. A method as in claim 20further comprising selecting the bandwidth of a plurality or bandpassfilters depending on the modulation of the signal being filtered by eachbandpass filter. 22-25. (canceled)
 26. An apparatus for transmittingdata on a fiber channel, the apparatus comprising: an input thatreceives a digital signal to be transmitted; a plurality of modulatorseach configured to accept a portion of the digital signal to betransmitted, and to modulate the portion of the digital signal accepted;a plurality of programmable mixers, each mixer coupled to the output ofone of the modulators to accept a modulated signal and mix it with amixer frequency, wherein the mixer frequency that is provided to themixer is programmable; a plurality of bandpass filters that operable tofilter the output of the mixers; and a summation unit operable tocombine the outputs of the bandpass filters into a single signal. 27.(canceled)
 28. An apparatus for transmitting data on a fiber channel,the apparatus comprising: an input that receives a digital signal to betransmitted; a plurality of modulators each configured to accept aportion of the digital signal to be transmitted, and to modulate theportion of the digital signal accepted; a plurality of mixers, eachmixer coupled to the output of one of the modulators to accept amodulated signal and mix it with a mixer frequency; a plurality ofprogrammable bandpass filters operable to filter the output of themixers, wherein each bandpass filter has a programmable bandpass; and asummation unit operable to combine the outputs of the bandpass filtersinto a single signal.
 29. An apparatus as in claim 26 wherein theapparatus is integrated within a single integrated circuit.
 30. Anapparatus as in claim 26 further comprising a transducer operable toapply the single signal to a fiber channel.